Haemophilus parasuis is an early colonizer of the upper respiratory tract of pigs. Non-virulent strains of this organism can be normally isolated from the nasal cavity, tonsil, and trachea of healthy pigs. Virulent strains of H. parasuis can also invade the host and cause systemic lesions characterized by fibrinous polyserositis, arthritis and meningitis.
Recent studies have demonstrated that non-virulent strains are more prevalent in the upper respiratory tract than virulent strains. However, even though the majority of pigs are colonized by non-virulent H. parasuis strains, they are not protected against systemic infection by virulent strains. It is widely recognized in the vaccine field that live vaccines may sometimes provide a higher degree and broader range of protective immunity than killed vaccines.
Controlled exposure to a low dose of live, virulent H. parasuis may reduce nursery mortality more efficiently than vaccination using commercial or autogenous killed products. However, exposure to live virulent bacteria poses significant risk of the very disease one is trying to protect against. Attenuation of the virulence of the vaccine strain in a way that is stable and also retains the immunogenicity of the live organism is therefore highly desirable to vaccine development.
Although neither the mechanisms involved in the protective immunity following controlled exposure, nor the identity of antigens which provide protective immunity, are clear at this time, it is known that protective immunity can be induced by such exposure. Moreover, pigs with controlled exposure to live H. parasuis are protected against homologous challenge through intratracheal and intraperitoneal routes, suggesting that exposed pigs develop local immune response (upper respiratory tract) as well as a systemic response. Live organisms undergo limited replication in the host at sites of infection and may thereby elicit additional arms of the immune response (e.g., local immunity) and/or augment the level of the response.
H. parasuis killed vaccines (commercial or autogenous) are known to generate poor cross-protection against different serovars. This may be due to incomplete expression of potential virulence factors when this organism is grown in vitro. Live organisms are more likely to express a wide variety of proteins and factors required for growth in vivo. Iron-regulated outer membrane proteins (IROMPS) are an example of potential virulence factors that may be expressed in vivo, but are rarely expressed in vitro. A modified live vaccine would be capable of expressing virulence factors when administered to a host. Most likely this product would induce a more involved immune response creating the potential for a safe, more efficacious vaccine.
The pork industry is in need of a safe and efficacious product capable of aiding the prevention of disease due to multiple serotypes of H. parasuis. 